The present invention relates to a novel ceramic heater for use in the manufacturing process of various kinds of electronic devices on which semiconductor silicon wafers as a substrate of semi-conductor devices, glass plates as a substrate of liquid crystal display panels and the like are mounted and heated in the course of the chemical vapor-phase deposition treatment, sputtering treatment and the like to form a thin film thereon or plasma etching treatment of the substrate surface. More particularly, the invention relates to a ceramic heater used in the above mentioned applications which is characterized by the greatly improved uniformity of the temperature distribution allover the surface thereof on which the workpiece such as semiconductor wafers and glass plates are mounted and heated. The invention also relates to a method for the preparation of such an improved ceramic heater.
Needless to say, the manufacturing process of various kinds of electronic devices almost always involves a step in which a semi-conductor silicon wafer as a substrate of semiconductor devices, glass plate as a substrate of liquid crystal display panels or the like is mounted on a heater and heated and kept at an elevated temperature suitable for processing of the substrate for film formation, etching and the like. A most conventional or traditional heater element used in such a heater is a high-resistivity metal wire wound in the form of a coil to serve as a resistance heater element. In view of the disadvantageous bulkiness of such a coiled heater element of a metal wire of high resistivity, proposals have been made, for example, in Japanese Patent Kokai No. 63-241921 and No. 4-124076 for a so-called ceramic heater which is an integral body comprising a substrate plate of an electrically insulating ceramic material and a layer of an electroconductive heat-resistant material formed on one surface of the substrate plate in the pattern of a heater element connected to an electric power source. The workpiece to be heated by the ceramic heater is mounted on the other surface of the substrate plate opposite to the surface on which the patterned heater layer is provided.
The ceramic heaters in the prior art mentioned above have several problems and disadvantages. When the ceramic heater is used by repeatedly heating up to a working temperature for the workpiece and cooling down to room temperature, for example, cracks are sometimes formed in the substrate plate and/or the patterned heater layer as a consequence of the thermal stress due to the repeated temperature elevation and lowering to cause circuit breaking or short circuiting. In some cases, separation or exfoliation may eventually take place between the substrate plate and the patterned layer of the electroconductive heat-resistant material as a consequence of the difference in the thermal expansion coefficients therebetween. A proposal has been made for the use of a fused silica glass plate as the substrate plate of a ceramic heater in view of the excellent resistance of fused silica glass plates against crack formation. A problem in such a ceramic heater by using a fused silica glass plate as the substrate is that, since fused silica glass is highly transparent to the light of visible to infrared region, the heat generated in the patterned heater layer is directly transmitted through the transparent substrate plate by thermal radiation so that the temperature of the surface of the substrate plate opposite to the patterned heater layer, on which a workpiece is mounted for heating, is more or less uneven or non-uniform following the temperature distribution in the patterned heater layer. This problem is more serious when the ceramic heater has large dimensions increased to comply with the requirement for processing of a workpiece of a larger and larger size because the unevenness in the temperature distribution on the surface of the ceramic heater directly influences the quality level of the products manufactured therewith and decreases the yield of acceptable products.